Inputs to any system vary with time, input device, and other factors. Systems for processing voice signals, for example, must cope with signals that vary because of a variety of reasons including different human speakers, distances from the audio microphone, transient fluctuations in the transmission facility, etc. In order to effectively process these inputs, it is known to equip the signal processing system with a variable gain stage driven by an automatic gain control (AGC) that compensates for the changing input levels.
Prior AGCs have experienced problems similar to the following:
Some AGCs increase gain when no signal is present, thereby degrading the output signal in the presence of input noise.
Some AGCs have poor attack and decay times, thereby taking too long to respond to increasing or decreasing signals.
Some AGCs with fast attack times change gain during a message, thereby presenting audible level differences to the operator.
Some AGCs use discrete components which change performance with time, temperature, and component variation, thereby decreasing performance.
Until recently, circuit designers have rarely utilized Digital Signal Processor (DSP) technology to solve these, and other. AGC problems, probably due to the substantial per-unit cost of early DSP units. As DSP units become more widely used, however, their per-unit cost will continue to fall and, ultimately, DSPs will be used in more AGC applications. As a result, there is a need for an improved AGC suitable for implementation by a DSP.